Circadian rhythm of nucleoli in rat superior cervical ganglion neurons: the two types of fibrillar centres and their quantitative relationship with the nucleolar organizing regions.

A quantitative stereological analysis was undertaken in nucleoli of rat superior cervical ganglion neurons. In this model, two types of fibrillar centres were observed: (1) small-type fibrillar centres were observed during the light span; and (2) a single large-type fibrillar centre occurred during the dark span near the smaller ones. The present data showed that the drastic increase in the mean volume of fibrillar centres during the dark span involved the occurrence and the progressive enlargement of one single large-type fibrillar centre and a marked rise in the number of small-type centres from 18 +/- 2 to 74 +/- 5 at 1500 h and 0100 h corresponding to light and dark spans, respectively. The total number of these small-type fibrillar centres per nucleolus increased with the total volume whereas their unit volume remained unchanged. This enabled us to establish some relationship with nucleolar organizing regions (NORs).